The present invention concerns a solution and method to expedite reworking molds for molding plastic parts, and in particular concerns a chemical solution and method for treating a mold half having mold surfaces to remove undesirable "heat discoloration" characteristics caused by heating the mold half while repairing or reworking the mold surfaces. Use of this solution and method reduces the time required for the repair or rework to a small fraction of that normally required, and also reduces the opportunity for human error.
Molded plastic parts offer a number of advantages over more traditionally manufactured sheet metal parts. For example, a complete part can often be made in a near finished condition including desired surface finishes in a single press cycle at very competitive costs. Further, plastic materials having special properties can be used Still further, the molds for making the parts can often be made with comparatively short lead times, and engineering changes can be made in the molds either before or after the molds are completed without the need to change an entire series of dies.
Molds for the plastic parts are characterized by a cover half and a corresponding ejector half adapted to open and close against each other, which ejector half and cover half include mold surfaces that cooperatively define a cavity in the shape of a part. These mold surfaces may be damaged if the mold halves are closed on a sharp object or on a previously formed part. Further, these mold surfaces wear and require maintenance, or may fracture over time with fatigue. Still further, engineering changes often need to be made for product improvement or fixes. Where the mold surface rework or repair requires adding material to one of the mold halves, the rework or repair can become unacceptably long, labor intensive, and expensive, as noted below.
In particular, adding material involves welding material to the mold surface, removing the excess material, and then "benching" and polishing the mold surface finish back to a desired surface finish. In order to minimize or eliminate weld lines, the mold half to be reworked is preheated before welding. However, during the process of heating the mold half, undesirable characteristics occur on the mold surface due to the particular steels used in making molds. These undesirable characteristics are visible as bluish haze known in the trade as "heat discoloration". These undesirable characteristics cannot be tolerated since they cause blemishes on the finished surface of a molded part, and further since they can cause non-uniform adhesion of the plastic to the mold surface, thus resulting in mold release problems as the molded plastic part is ejected from the mold.
Historically, these undesirable "heat discoloration" characteristics have been removed by many hours of tedious work in the form of "benching" and manually polishing to delicately abrasively bring the mold surface back to the required surface finish. This required surface finish is comparable to that of jewelry, and often must be accurate to within thousandths of an inch or better. This benching method of removal can actually take hours and even days of skilled labor time since it requires careful tedious polishing of the mold surfaces, including areas that were not repaired or reworked. Further, damage can occur by over-polishing sensitive areas of the molds, such as by over-polishing shutoff areas on the mold (i.e. areas that prevent plastic from leaking out of the cavity during the molding process) thus causing flash, or by over-polishing details on the mold surface thus causing the details to become "washed out". Further, ribs and other deep crevices or sharp corners may be difficult to properly polish, or even impossible to reach, such that the heat discoloration characteristics cannot adequately be removed, leading to manufacturing problems or unresolvable part defects. Yet, such tedious, costly benching and polishing is believed to be universally practiced in high quality tool and die shops and generally through the tool and die industry.
Thus, a method and solution is desired to solve the aforementioned problems to avoid a substantial amount of tedious and costly benching and polishing.